Gasoline typically includes a mixture of hydrocarbons ranging from higher volatility butanes (C4) to lower volatility C8 to C10 hydrocarbons. When vapor pressure increases in the fuel tank due to conditions such as higher ambient temperature or displacement of vapor during filling of the tank, fuel vapor flows through openings in the fuel tank. To prevent fuel vapor loss into the atmosphere, the fuel tank is vented into a canister that contains an adsorbent material such as activated carbon granules (“evap” canister).
The fuel vapor is a mixture of the gasoline vapor (referred to in this description also by its main component, hydrocarbon vapor) and air. As the fuel vapor enters an inlet of the canister, the hydrocarbon vapor is adsorbed onto activated carbon granules and the air escapes into the atmosphere. The size of the canister and the volume of the adsorbent activated carbon are selected to accommodate the expected gasoline vapor generation. After the engine is started, the control system uses engine intake vacuum to draw air through the adsorbent to desorb the fuel. The desorbed fuel vapor is directed into an air induction system of the engine as a secondary air/fuel mixture. One exemplary evaporative control system is described in U.S. Pat. No. 6,279,548 to Reddy, which is hereby incorporated by reference.
When the gasoline tank is filled, fuel vapor accumulates in the canister. The initial loading may be at the inlet end of the canister, but over time the fuel vapor is gradually distributed along the entire bed of the adsorbent material. After the engine is started, a purge valve is opened and air is drawn through the canister. The air removes fuel vapor that is stored in the adsorbent material.
One problem encountered by such a system has been vapor breakthrough, or hydrocarbon emissions from the vented vapor adsorption canister, which is often referred to as canister bleed emissions. Such bleed emissions may be, for example, about 20 mg hydrocarbons per day. Co-pending U.S. patent application Ser. No. 10/303,556 describes a method and system for evaporative emission control in which such bleed emissions are adsorbed by activated carbon fibers. The system may be used in a conventional automotive vehicle having an internal combustion engine or in a hybrid vehicle that includes both an internal combustion engine and an electric motor. The activated carbon fiber material can desorb the adsorbed hydrocarbons when purged with air without being heated.
In a hybrid vehicle, however, the internal combustion engine is turned off nearly half of the time during vehicle operation. Because the purging takes place only during operation of the internal combustion engine when the desorbed vapor can be consumed in engine combustion, the evap canister purging with fresh air occurs less than half of the time in a hybrid vehicle. A hybrid vehicle generates nearly the same amount of evaporative fuel vapor as does a conventional vehicle, however, with the result that the lower purge rate of the hybrid vehicle is not sufficient to clean the adsorbed fuel out of the evap canister, resulting in higher evaporative bleed or breakthrough emissions. FIG. 1 demonstrates the difference in bleed emissions, using air purging in a conventional vehicle with only an internal combustion engine and in a hybrid vehicle. With only half the air purge rate, the hybrid vehicle had an unacceptable 27 mg of bleed emissions while the conventional vehicle had only 4 mg of bleed emissions using the system for evaporative emission control of co-pending U.S. patent application Ser. No. 10/303,556. It would thus be desirable to modify this system for use in a hybrid vehicle to reduce bleed emissions.